The present invention relates to the field of antenna systems. More specifically, the present invention relates to adaptive antenna systems.
An antenna system is a port through which radio frequency (RF) energy is coupled from the transmitter to the surrounding environment and, in reverse, to the receiver from the surrounding environment. The manner in which energy is transmitted into and received from the surrounding environment influences the efficient use of spectrum, cost of establishing networks, and quality of service provided by these networks. One class of antenna systems that has received a great deal of attention for use in wireless communication and radar applications is that of adaptive antenna systems. An adaptive antenna system attempts to augment signal quality of a radio-based system by optimizing its radiation and/or reception pattern automatically in response to the signal environment.
One exemplary adaptive antenna system is a phased-array antenna. Phased-array antennas are built from a large number of small antenna elements, the amplitude and phase of which can be controlled individually with electronic modulators which direct and redirect their focus to maximize the strength of a transmitted signal. To electronically steer a beam, an electromagnetic signal to be transmitted is split and distributed to each of the antenna elements which shift the phase of the signal based on their position in the array and the desired beam pointing direction. Received electromagnetic signals are likewise phase-shifted and combined.
Phased-array antennas, provide great agility, fast tracking, and the ability to use multiple antenna beams simultaneously. However, a disadvantage of phased-array antennas for large scale applications is physical size and weight of the beamforming network, which contains a modulator for each antenna element (typically hundreds to thousands). In addition, conventional phased-array antennas are very expensive, limiting their use to military and other high value applications.
In some applications, Fresnel zone plate antennas can be utilized as a less costly alternative to the more complex and expensive phased-array antennas. Fresnel zone plate antennas may be configured as either lens or reflector antennas, and generally include two elements, a transmission or reflection zone plate, respectively, and a feeder element. The feeder element (for example, an open waveguide, horn dipole, etc.) is typically placed at a primary focus of the zone plate. The Fresnel zone plate converts a spherical wave radiated by the feeder element into a plane wave (transmitting antenna) or an incident plane wave into a spherical wave focused at the feeder element (receiving antenna).
A reflector Fresnel zone plate antenna, i.e., a zone plate configured as a reflector antenna, typically has alternating transparent and metallic rings, or zones) that are coarsely spaced at the center (producing a small diffraction angle) and finely spaced at the outside (producing a large diffraction angle) so as to concentrate electromagnetic waves at a focal point in front of the zone plate. The metallic rings reflect an electromagnetic wave, which constructively interferes in front of the Fresnel zone plate at the focal point, whereas the transparent rings are nulls. The exact pattern, i.e., radii, of the rings determines which frequency or wavelength is concentrated, and exactly where it will be concentrated. As known to those skilled in the art, the radius of each ring or zone, RN, can be given by:       R    N    2    =                    (                  f          +                                    N              ⁢                              xe2x80x83                            ⁢              λ                        2                          )            2        -          f      2      
where RN is the radius of the Nth boundary, N is the zone number, f is the focal length of the zone plate (i.e., the distance to the point of constructive interference), and xcex is the wavelength of the electromagnetic wave. Thus, to generalize, the Fresnel zone plate antenna acts as a reflector with a focal length of xe2x80x9cfxe2x80x9d for an electromagnetic wave with a wavelength of xe2x80x9cxcexxe2x80x9d. A reflector screen may be placed one quarter wavelength behind a Fresnel zone plate so that all zones of the zone plate may be used, rather than just alternating zones. That is, through the use of the reflector screen, rays passing through the transparent rings reflect from the reflector screen and further contribute to the energy at the focal point.
Reflector Fresnel zone plate antennas may be fabricated by laying down metal rings on a substrate to form the shape of the antenna patterns. Alternatively, the construction of a Fresnel zone plate may be achieved by other manufacturing processes such as machining out of solid metal, stamping out of a thin metal sheet, molding and subsequently metallizing a plastic material or by vacuum forming plastics.
Unfortunately, such rigid manufacturing techniques result in Fresnel zone plate antennas that are not adaptive to changing frequencies and directions of electromagnetic waves. That is, a Fresnel zone plate pattern is manufactured for transmission and/or reception of relatively narrow bandwidth electromagnetic waves, which can only be directed in a specific beam direction.
Accordingly, what is needed is an economical antenna system that is dynamically adjustable to change frequency at which the antenna will transmit or receive, and is dynamically adjustable to change direction in which a received or transmitted electromagnetic wave is steered.
Accordingly, it is an advantage of the present invention that an adaptive antenna system is provided.
It is another advantage of the present invention that the adaptive antenna system is dynamically adjustable for adapting to changing frequencies and directions of transmitted and received electromagnetic waves.
Yet another advantage of the present invention is that an adaptive antenna system is provided that is economical to manufacture.
The above and other advantages of the present invention are carried out in one form by an adaptive antenna system that includes a programmable reflection surface for reflecting an electromagnetic wave and a controller in communication with the programmable reflection surface. The controller is operable to write a pattern into the programmable reflection surface in accordance with a frequency of the electromagnetic wave, the pattern including a reflective region and an absorptive region.
The above and other advantages of the present invention are carried out in another form by an adaptive antenna system that includes electronic paper for reflecting an electromagnetic wave, said electronic paper being electrically writable and erasable, and a controller in communication with the electronic paper. The controller is operable to write a pattern into the programmable reflection surface in accordance with a frequency and a direction of the electromagnetic wave, the pattern including a reflective region and an absorptive region. A feeder element is outwardly spaced from a receiving side of the electronic paper such that a location of the feeder element relative to the electronic paper defines a focal point for said pattern.